I am migrating from Final Cut Pro and like Premiere Pro CS 5 well, but I would like to know how to open a close view of an audio track and place keyframes on it to precisely control when volume levels are applied. I can open a whole clip in the "source" window, but the pen tool doesn't let me add a keyframe in that window.

I may not understand what your are referring to, but wouldn't increasing the height of the audio track on the timeline and widening the timeline horizontally give you the resolution so that you can use the pointer to place your audio keyframes?

Though it gets down to personal preferences, I find that doing Keyframing of Audio Volume (and many other Effects, both Fixed and added) is much, much easier in the Effects Control Panel. Select your Clip, and Open the Effects Control Panel. There you will immediately see the Fixed Effects, with Volume for the muxed Audio in a Clip. Add and adjust your Keyframes there. Note the little horizontal zoom slider, expand the little Timeline in that Panel. If you do not see the Timeline in the Effects Control Panel, hit the >> in the upper-right to expose it.

For some other methods of adjusting Volume/Gain, see this ARTICLE. I also use the Audio Mixer to automate my attenuation, but that applies Track Keyframes, that stay with the Track, regardless of what you do to Clips on that Track.